Temperature-sensitive mutants of influenza A virus: response of children to the influenza A/Hong Kong/68-ts-1(E) (H3N2) and influenza A/Udorn/72-ts-1(E) (H3N2) candidate vaccine viruses and significance of immunity to neuraminidase antigen.

One of two slightly different influenza A/ts-1[E] recombinant candidate live vaccines was given intranasally to each of 23 young children. Twelve of 15 children who had no serum HI antibody but who did have serum ANAB at the time of administration became infected and 1 had mild rhinitis. All eight who lacked both types of antibody became infected and they shed virus in higher titer and for longer than the former group; five had rhinorrhea and five had mild fever. These findings suggest that serum ANAB plays a part in modulating influenza virus infection and that the full expression of virulence of these or other attenuated influenza vaccines may be manifest only in individuals lacking both HI antibody and ANAB. These particular candidate vaccine strains appear to be attenuated for older children (who have some prior experience with influenza A as demonstrated by serum ANAB), but the occurrence of fever in over half who had no prior experience indicates that they would not be acceptable for a vaccine in wide-spread use.